*{margin: 0; padding: 0;}
a{text-decoration: none;}
kbd{border: 1px solid red;width: 4em;height: 4em;display: inline-block;
    text-transform: uppercase;position: relative;/*将小写变成大写*/}
kbd > button{
    position: absolute;
    left: 0;
    bottom: 0; 
    display: none; /*刚开始不显示，和下面的hover连起来展现效果*/ 
}
kbd:hover > button{
    display: inline-block;
}
body{
    background:blue url(./wall.jpg) no-repeat center center;
    background-size: cover;
}
main{text-align: center;
     height: 100vh;/*viewport height*/
     display:flex;
    align-items: center;
    justify-content: center;
}
.wrapper{/*display: inline-block;*/ /*将大的div变成内联元素*/
      background:rgba(255,255,255,0.3);
      border-radius: 10px;
}   
.wrapper > div:nth-child(2){ /*div:nth-child(2)表示父元素中子元素为第二的并且名字为div的标签被选中*/
    margin-left: -20px;
}    
.wrapper> div:nth-child(3){ /*用class 的话前面要加. 用id的话前面要加# */
    margin-left: -90px;
} 
.key{
    width: 50px;
    height: 40px;
    border: none;
    color: #c5c5c5;
    border: 1px solid #373737;
    border-radius: 6px;
    background: linear-gradient(to bottom, #292929 0%,#111111 100%);
    box-shadow: 0 0 0 1px #1a1b1c,0 0 0 2px #1f2020 ,0 3px 0 2px #080808 ;
    position: relative;
    font-size: 17px;
    font-family: Helvetica;
} 
.row{
    margin: 15px;
}
.row > .key{
    margin:0 10px;
}
.key img{
    width: 18px;
    height: 18px;
    border-radius: 3px;
    position:absolute;
    left:33%;
    bottom: 0;
}
.key .text{
    position: absolute;
    left: 4px;
    top: 2px;
}
